A friction clutch has been widely used as a clutch for a transmission of a vehicle such as a car, a railway vehicle or the like. A frictional material for use in a friction clutch has been obtained in a technique where a fiber base material such as natural pulp fibers, organic synthetic fibers, inorganic fibers, or the like, and a filler or a frictional adjustment agent such as diatomaceous earth, cashew resin, or the like, are formed by wet sheet making, impregnated with a thermosetting synthetic resin such as phenol resin, or the like, dried and compression-molded.
Such frictional materials are classified into two types, including a dry frictional material that is used in a dry state and a wet frictional material that is used in oil as described above. Each of these has been suitably developed for their respective applications.
Although various kinds of synthetic thermosetting resins have been used in such frictional materials, phenol resin has been widely used for a variety of reasons such as heat resistance, abrasion resistance, handleability, price, etc. The phenol resin for use in a frictional material is modified with various kinds of resins in order to improve the characteristics thereof. There are many kinds of useful modified phenol resins. For example, linear organopolysiloxane is bonded with novolac phenol resin so as to improve elasticity, tension propagation and tensile strength as described in JP-A-55-92738 (the term "JP-A" as used herein means an unexamined published Japanese patent application).
However, when phenol resin is modified with linear organopolysiloxane in this manner, its softening point drops to deteriorate the solvent resistance. In addition, cracking occurs in organopolysiloxane to thereby produce siloxane of a low molecular weight which undesirably imparts slip characteristics to a frictional material. Therefore, according to another proposal, in order to obtain a binder having superior heat resistance and abrasion resistance, a frictional material containing a modified phenol resin as described above is manufactured using a binder, the main component of which is phenol resin modified with an organopolysiloxane containing a R.sup.1 SiO.sub.1.5 unit (where R.sup.1 represents the same or different, substituted or unsubstituted monovalent hydrocarbon group having from 1 to 8 carbon atoms) and/or an SiO.sub.2 unit in a molecule thereof as described in JP-A-61-192711.
In addition, according to another proposal, in a clutch facing with component substances such as fiber substances, abrasion resistant powder, metal wires or metal powder and binders, another binder is also used together with a conventional binder such as phenol-formaldehyde resin, melamine-formaldehyde resin, melamine/phenol-formaldehyde resin, SBR, NBR, natural rubber, or the like, so that silicone resin is present in an amount of from 0.1 to 10 weight % of the clutch facing as described in JP-A-60-28484.
With this arrangement, a clutch facing which resists moisture absorption can be obtained. Although JP-A-60-28484 teaches that the silicone resin used therein is not limited so long as it is a silicone resin having a moisture barrier effect, JP-A-60-28484 only exemplifies dimethylsiloxane-system silicone resin, and fails to describe the technical content thereof in detail.
In recent automatic transmissions for cars, energy and environmental considerations require a slip control on the friction clutch or a reduction in ATF (automatic transmission oil) friction in order to reduce fuel expense and to provide a smoother gear change.
However, the use of a slip control or reduction in ATF friction necesitate a frictional material having higher heat resistance, durability and strength. Conventional clutches are also disadvantageous in that slipping appears during engagement due to scorching or separation of a friction surface, or a decrease of its friction coefficient to thereby cause burning. Therefore, there is a need to solve this problem.
Although a variety of techniques may be considered as measures for solving these problems, including increasing the friction area by increasing the number of frictional plates or enlarging the diameter of a clutch, increasing the clutch control oil pressure, etc., each of these methods undesirably increases the size of the automatic transmission. Thus, there is a need in the art to provide another solution to the above described problems which does not adversely affect other aspects of the transmission.